For purposes of this discussion, machining a workpiece involves fixturing the workpiece in relationship to a machine tool. This is typically achieved by clamping a workpiece to a fixture which is in turn, mounted to the work table of a machine.
Machining techniques are available in several forms, depending on the workpiece design, material, specification, and application. Generally, as used herein, a machine will have a table for supporting a workpiece (or a fixture-supported workpiece), and a spindle for rotating a cutting tool that removes material from the workpiece. In the main, hereinafter, the use of a face mill cutting tool for machining a surface to be flat is discussed.
Depending on the workpiece application and specification, it may be required to machine (rough or finish) a surface (of a workpiece) and obtain flatness within a certain tolerance. Also, it may be specified that two opposite surfaces (e.g., top and bottom) are parallel to one another. As used herein, the concept of parallel, as applied to two faces of a workpiece, means that the plane established by a one of the two opposite faces is spaced apart from the plane established by an other of the two opposite faces by a uniform distance.
For example, an automatic transmission valve body may require that two opposite surfaces be machined so as to be flat (planar) and parallel to one another. The traditional manner of achieving this objective is to clamp the workpiece in a fixture, machine the one surface, then turn the workpiece over and re-clamp it--using the previously machined flat surface as a reference for machining the second (opposite) surface to be parallel. Typically, in order to machine (e.g., face mill) a workpiece, three button-like cast locators are disposed on the fixture base to support the opposite surface of the workpiece. Said opposite surface may be a previously-machined surface. The three cast locators act as "pedestals" or "legs" to establish a plane for machining the surface of the workpiece. Using such cast locators (disposed between the fixture base and the workpiece) can provide various sources of error in machining the second surface. For example:
a. chips or dirt between the workpiece and the fixture base (i.e., the cast locator buttons) to which the workpiece is mounted can result in the workpiece tipping--the result of which is that the second surface, when machined, will not be parallel to the first surface;
b. clamping (e.g., automatic clamping) the workpiece to the fixture base may distort the workpiece (depending upon its shape and material, or stiffness). This is especially noticeable when clamping down on the workpiece at a location that is not directly over the cast locators, which will tend to bend (distort) the workpiece. Such distortion of the workpiece will result in inaccurate machining--the result of which is that when the workpiece is un-clamped, the second surface will not be flat and will not be parallel to the first surface; and
c. any surface irregularities (e.g., waviness, shingling, etc.) in the already-machined (albeit flat) first surface may cause an error in the machining of the second surface. This is primarily due to the fact that the clamping location tends to be fixed, while the location of "hills" and "valleys" on the previously-machined surface tend to be indeterminate in their location. Such surface irregularities, and clamping thereto (onto cast locators), may cause an unacceptable mislocation of the workpiece when performing the machining operation.
In the main hereinafter, the workpiece to be machined may be a casting of an automatic transmission valve body (aluminum material, thin, lightweight, and fragile). The valve body has a number of conduits (channels) extending into both the top and bottom surfaces thereof, for conveying automatic transmission fluid. The drive ratio of the automatic transmission is selected by the routing of these fluids through the various conduits disposed in the valve body.
Such a casting is typically irregularly shaped, has a top surface, a bottom surface opposite the top surface, and an edge surface extending around the periphery of the valve body. The thickness (as measured from the top surface to the bottom surface) of the casting varies (by way of example) from a maximum of 17/8 inch to a minimum of 11/16 inch, which for purposes of this discussion is considered to be a thin workpiece.
Because the workpiece is made of aluminum, irregularly shaped, thin and therefore fragile, it is very difficult to accurately mill the workpiece flat to a tight specification, for example to within one thousandth of an inch (0.001"), and to hold parallelism between the two opposite surfaces to within one thousandth of an inch. The workpiece tends to deflect during milling, introducing anomalies into the surfaces.
When machining thin aluminum parts that require opposite parallel surfaces to mate with other parts (e.g., fluid control modules), it is common to first use cast locators (e.g., "legs", as described above) for rough or finish machining of the first machined surface (e.g., the top surface), and then to locate from three points (again, via cast locators disposed between the fixture base and the workpiece) on the previously-machined surface to surface machine the second, opposite surface (e.g., the bottom surface).
On some workpieces, notably automatic transmission control valve bodies, both opposite surfaces must be machined over their entirety. This makes it impossible to clamp directly over a vertical cast locator between the fixture base and the first machined face. When clamps exert a mechanical force through the workpiece anywhere other than directly over a cast locator, they will tend to distort the workpiece. Even though the distortion may be less than 0.002", such a distortion could be sufficient to cause the part to be out of tolerance in either flatness or parallelism.
U.S. Pat. No. 4,964,766, incorporated by reference herein, is representative of a fixturing apparatus for locating, orienting and holding a workpiece during a milling operation, having a workpiece-specific mounting provided with means for accurately and reproducibly positioning the workpiece within the fixture. The workpiece specific mounting has a workpiece specific pattern of clamping, vibration dampening and supporting devices to hold the workpiece during the milling operation. This patent addresses holding the workpiece for minimizing distortion and permitting for accurate, reproducible location and orientation of successive workpieces of the same geometry on the apparatus for the same milling operations that is achieved quickly and positively and vibration and workpiece distortion are minimized during the milling operations. However, the patent does not specifically address the techniques of the present invention.